Oliver Queen has been hit with perhaps his biggest blast from the past.

As The CW’s Arrow flew into the holiday break, Oliver was distraught to discover that the Mirakuru serum he first encountered during his island days has found its way to Starling City and into the hands of a mystery man hiding behind a skull mask (aka “Brother” Sebastian Blood).

Yet just like that, the grunts exhibiting superhuman strength that Arrow had grappled with in recent weeks made all the sense in the world. And yet… didn’t. After all, how did the serum that first infected his island pal Slade Wilson find its way to Oliver’s hometown?

To get answers, as the season resumes Wednesday at 8/7c, “Oliver is obsessed with whomever the guy in the skull mask is,” Stephen Amell tells TVLine. “Like, he’s losing his mind, freaking out because Mirakuru has a tie to the past for him” — and a very haunting one, at that.

“As we’re learning, all the stuff that happened on the island and [on the freighter] was not good,” Amell reminds. “It was stuff that he thought he had buried in the past. So for it to crop back up again is not a good sign.”

And though we, the viewers, are now privy to the connective tissue between Oliver’s past odyssey and Arrow’s present-day crisis, Amell says, “Oliver has no idea” that Slade, too, is in Starling City.

Elsewhere on the action front, Amell says it’s “extremely exciting” to play with some Batman DNA and bring an al Ghul to town, when Spartacus‘ Katrina Law guests as Ra’s daughter Nyssa in Episode 13, “Heir to the Demon,” and he promises “a really, really good” fight sequence in Episode 14 — one that required him to “slide down a bunch of stairs,” leap over a railing and tussle with toughs who “really aren’t a match for me, so I have to find creative ways to take them down.”

Oliver’s personal life portends to be just as bumpy a ride, when, as revealed in TVLine’s Winter Preview special, onetime lady love Laurel (played by Katie Cassidy) hits rock bottom with her drug and booze abuse.

“Oliver’s going to play a part in [that arc], but not in the way you might think,” Amell teases. “Oliver has always been very protective of Laurel, and her moment of clarity, so to speak, happens after an encounter with Oliver unlike one that we’ve ever seen on the show.” (With reporting by Scott Huver)